1. Field of the Invention
The present invention relates to a fluidized bed processor such as a fluidized bed dryer, coater, or agglomerator utilizing the application of microwave energy. A microwave generator is connected with a fluidized bed processor to direct microwave energy into the region of fluidization in variously-oriented directions to create a plurality of standing wave modes within the processor through which the fluidized particles move in random directions.
2. Description of the Prior Art
Fluidized bed processors, of the batch type, are well known in the prior art. Such fluidized bed processors are used for a variety of drying, coating and agglomerating operations. The fluidized bed processors include a fluidized bed vessel having a fluidizing gas inlet and a fluidizing gas outlet to enable the gas to fluidize the product being treated. One such fluidized bed processor, particularly useful for laboratory operations, is the Uni-Glatt (TM) processor wherein the fluidized bed vessel includes a product container, or loading bowl, for receiving the product to be treated, an expansion chamber disposed immediately above the product container, and a filter chamber connected to the opposite end of the expansion chamber. A product retaining screen is provided at the bottommost end of the product container to support the product to be fluidized and defines an air inlet for allowing the fluidized gas to be directed upwardly to fluidize the product to be treated. The gas is drawn upwards through the vessel and exits the fluidizing gas outlet at the upper region of the filter chamber. The fluidized bed is formed within the product container and the expansion chamber. The filter chamber includes filters for catching any particles that may be blown upwards and includes a mechanical shaking apparatus or other means to shake or vibrate the filter bag to remove any particles embedded therein during the operation. When the product treatment is completed, the processor fans are stopped, the product container is disconnected from the expansion chamber, and the treated product is removed.
Fluidized bed processing, such as fluidized bed drying, has certain disadvantages. The fluidizing gas, typically heated air, serves to evaporate moisture from the particle surface. The drying time is dependent upon the mass of air flow and the amount of energy absorbed into the particulate material. During the initial stages of drying, moisture is evaporated from the surface of the particles relatively rapidly. However, the drying time falls after the surface moisture is removed. During this second phase of drying, water or moisture removal is dependent upon capillary or diffusional action. This requires increasingly more energy as the percent of moisture decreases or, stated otherwise, as the distance from the dried surface to the moisture increases. The product, which is usually a poor heat conductor, requires greater and greater energy input (higher temperatures) to maintain an adequate level of heat transfer to drive the liquid out to the surface of the particles. Thus, in the later stage of fluidized bed drying, the drying rate falls due to the relatively poor energy efficiency.
The use of fluidized bed dryers, in combination with microwave energy sources are known in the art. See, for example, U.S. Pat. Nos. 3,528,179 and 4,126,945. These systems seek to take advantage of microwave energy which penetrates, to a great extent, into the product. Microwave energy serves to drive the moisture in the center of the product to be treated toward the surface. Once driven to the surface, the fluidizing gas provides for removal of the evaporating surface moisture.
The above-mentioned prior art patents that utilize microwave energy provide that the fluidizing bed vessel be shaped in the form of a waveguide, i.e., a traveling wave applicator. Situated substantially in the center of the waveguide, or slightly below the center-line of the waveguide, is a product retaining screen which supports the product to be fluidized. Microwave energy is applied at one end of this waveguide and microwaves are propagated both above and below the product screen along the length of the waveguide. The opposite end of the waveguide includes load tubes to absorb microwave energy not absorbed by the product.
The disadvantages of the above-described systems, which require the entire fluidized bed vessel to be formed as a waveguide, is that all of the microwaves are propagated along the path of the product and any remaining energy which has not been absorbed by the product is absorbed at the end of the microwave transmission path by the load tubes. Thus, no reflected waves occur and no standing waves exist. Such applications are inefficient since energy not absorbed by the load is lost in the load tubes. Moreover, in these prior art types of traveling wave applicators, the product to be treated is positioned in a region of maximum electric field which assists in maximizing absorbtion but limits design flexibility because the product must remain at a relatively precise location in the waveguide. Since the maximum electric field occurs substantially in the center of the waveguide, the product supporting screen is positioned near this center line and thus the area beneath the screen must also serve as part of the waveguide. As a result, much of the microwave energy propagated along the waveguide length is propagated below the screen and, of course, below the product being fluidized thus providing for an inefficient use of the microwave energy.
The remodelling of a conventional batch fluidized bed dryer to accommodate the use of microwave power in addition to hot air is mentioned in Rzepecka, M. H., Hamid, M. A., and McConnell, M. B., "Microwave Fluidized Bed Dyrer", Microwave Power Symposium, 10th Ann. Proc., University of Waterloo, Ont., May 1975, pp. 297-299. The article refers to a circular cylindrical drying chamber and notes that circular cylindrical cavities have been used as multimode resonators in microwave power processing. The arrangement of the drying chamber vis a vis the microwave energy source is not described.